Electric power is commercially available in a variety of alternating current formats, either in single phase or multiple-phase, 50Hz or 60Hz, and up to 550V.
Power consumption is directly proportional to the vector product of voltage and current. If voltage and current are in phase, real power consumption is readily calculable. However if the current shifts relative to the voltage, the apparent or reactive power consumption is more difficult to measure using conventional methods. Accordingly, hydro utilities install a special meter for measuring the angle between the voltage and current. If the reactive power shifts beyond an acceptable amount, normally 5-10%, the customer is penalized.
The amount of phase shift is dependent upon the type of load being driven by the electric power. A predominantly reactive load will shift the current relative to the voltage in a positive sense whereas a predominantly capacitive load will shift the current in the negative sense.
In a typical example, when three phase 550V AC is supplied to a 550V 150 hp loaded motor, the total amperage that the motor is receiving is in excess of 140 amps. This current must not lag more than 5-10% behind the voltage, otherwise penalty charges are imposed by the utility. The inductive loads inherent in the windings of the motor will shift the current, up to 90.degree., depending on the actual inductive, capacitive and resistive components in the system.
Traditional techniques that have been used in the past have tried to make use of transformers and capacitors to try to phase correct this current shift. However, the current shift and the size of the transformers and capacitors necessitates a very large and cumbersome and hence expensive solution. For example, a Westinghouse system which can only handle about a 20 hp motor, has a size in excess of 4 feet high and 2 feet by 2 feet in base area.
Still other devices exist such as those found in the following U.S. Pat. Nos.; 4,894,763, 4,961,130, 4,977,492, 5,043,857, 5,055,939, 5,045,991, 5,111,374. What these devices attempt to do is through either other inductors, capacitors or combinations of both, some being computer controlled, cause the current to be further delayed so that it appears during the next sinusoidal voltage waveform in the correct position. Hence these devices are generally large and complex.
Heretofore, no single type of device has been suitable for all types of loads, whether reactive, capacitive or variable between the two.